This invention relates to ceramic capacitors having a medium temperature firing dielectric of lead-lanthanum zirconate-titanate and glass, and more particularly to such capacitors wherein the dielectric contains bismuth and the ceramic is doped and reacted with silver. The term doped as used herein is intended to mean reacted and not simply mixed with.
Glass containing ceramic dielectric materials are known as being capable of densification at medium firing temperatures below 2200.degree. F., whereas ceramics without glass generally require a firing temperature of greater than 2200.degree. F. and often at least 2450.degree. F.
Lead-zirconate-titanate ceramics have been employed for many years as electromechanical transducers and more recently lead-lanthanum zirconate-titanates have found use in electro-optic transducers. For these purposes, the ceramic is formulated so as to form a ferroelectric body, which formulations are unsuitable for making electrical capacitors that must exhibit a relatively stable dielectric constant as a function of the applied electric field. It is common practice to fire these lead-containing ceramics in a closed crucible that has a quantity of a lead compound contained therein for providing a rich lead atmosphere and preventing escape of the volatile lead from the ceramic formulation.
It is often desirable to have a capacitor that exhibits a low temperature coefficient of capacitance (TCC) and a low voltage coefficient of capacitance (VCC). It is further desirable that the dielectric material have a high dielectric constant to minimize the required size of a capacitor having a given capacity value. It is well known that the addition of glass to a ceramic body will smooth the TCC at the expense of reducing the dielectric constant.
It is an object of this invention to provide an improved ceramic capacitor that exhibits a low TCC and a high volt-microfarad product for any given capacitor size.
It is a further object of this invention to provide a low cost monolithic capacitor suitable for use at high a.c. and d.c. voltages.
It is another object of this invention to provide an anti-ferroelectric lead-lanthanum zirconate-titanate dielectric body having a low TCC and a high dielectric constant.